Fan Affinity Laws

Often times a fan’s speed or impeller diameter will be changed. If the fan is a centrifugal fan, then the change in performance of the fan can be predicted quickly through the affinity laws.

First if the impeller diameter is held constant and the speed of the fan is changed, then flow rate varies directly with the speed, available pressure varies with the square of the speed and the power use varies with the cube of the speed.

Second if the speed is held constant and the impeller diameter of the fan is changed, then flow rate varies directly with the diameter, available pressure varies with the diameter of the speed and the power use varies with the cube of the diameter.